The subject matter described herein relates generally to electric power systems, and more specifically, to voltage regulation of an auxiliary electric power system for a wind turbine.
Many known renewable energy facilities are coupled to an electric utility grid. At least some of these known renewable energy facilities include wind turbines. Generally, a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. At least some of the known wind turbines are physically nested together in a common geographical region to form a wind turbine farm, sometimes referred to as a wind farm. Variable speed operation of the wind turbine facilitates enhanced capture of energy when compared to a constant speed operation of the wind turbine. However, variable speed operation of the wind turbine produces electric power having varying voltage and/or frequency. A power converter may be coupled between the wind turbine's electric generator and an electric utility grid. The power converter receives the electric power from the wind turbine generator and transmits electricity having a fixed voltage and frequency for further transmission to the utility grid via a main power transformer. Typically, the high side of the main transformer is coupled to the grid and the low side is coupled to the power converter. Conversely, for those periods when the generator is not is service, electric power may be provided from the grid through the high side of the main power transformer to the low side of the main power transformer and then through the power converter.
Known wind turbines include auxiliary support equipment that facilitates operation of such wind turbines, for example, blade pitch drive motors, lubrication pump motors, and wind turbine and power converter control systems. In at least some wind turbine facilities, when the wind turbine generator is in service, such auxiliary support equipment receives at least a portion of electric power generated by the wind turbine generator through an auxiliary power transformer. The high side of the auxiliary power transformer is coupled to the low side of the main transformer and the low side of the auxiliary transformer may be coupled to the auxiliary support equipment. When the wind turbine generator is not in service, such auxiliary support equipment receives electric power from the grid through the main transformer and the auxiliary transformer. Moreover, such auxiliary support equipment typically has a predetermined voltage tolerance range. For example, at least some known support equipment may have a tolerance range that extends from 90% of nameplate voltage to 110% of nameplate voltage.
In addition, many known electrical grids have voltage tolerance ranges that facilitate reliable electric power transmission and distribution over a wide variety of operational conditions to serve a broad market. For example, many known electrical grids include a grid voltage tolerance range that extends from less than 90% of nominally rated voltage to greater than 110% of nominally rated voltage. As such, many known wind turbines include auxiliary support equipment that is designed to operate within a voltage window that is not fully complimentary to the voltage window of the associated electrical grid. Exceeding the voltage tolerance ranges of the equipment may impair the operation of the equipment. Substituting, or replacing, such auxiliary support equipment with specialized equipment having broader electric power tolerances may be costly and may require an extended period of time that the wind turbine must be removed from service.
Moreover, as more renewable energy sources are coupled to the grid, the requirements for ride through are becoming increasingly stringent. Specifically, in at least some jurisdictions, the temporal requirements and transient voltage amplitude ranges for sustaining ride through are being extended. The wind turbine may not be able to operate through certain grid events occurring on the high side of the transformer, since wind turbine control devices require a finite period of time to sense the event, and then make adjustments to wind turbine operation to take effect after detecting such grid event. Therefore, in the interim period, the wind turbine may sustain wear and/or damage due to certain grid events. Such grid events may include electrical faults that, under certain circumstances, may induce grid voltage fluctuations that may include low voltage transients with voltage fluctuations that approach zero volts. Moreover, such grid events may include grid voltage fluctuations that may include high voltage transients with voltage fluctuations that may approach and/or exceed equipment ratings. In addition, such grid events, under certain conditions, may induce frequency excursions as well.
At least some known protective devices and systems facilitate continued operation during certain grid events. For example, for grid transients such as short circuits, a low, or zero voltage condition on the grid may occur. Under such conditions, such known protective devices and systems define a low and/or a zero voltage ride through (LVRT and ZVRT, respectively) capability. Such LVRT/ZVRT capabilities facilitate operation of the power converters of individual wind turbines and wind turbine farms to transmit reactive power into the utility grid. Such injection of reactive power into the grid facilitates stabilizing the grid voltage while grid isolation devices external to the wind farm, such as automated reclosers, will open and reclose to clear the fault while the LVRT/ZVRT features of the wind turbines maintain the generators coupled to the utility grid. Moreover, for high voltage grid conditions, such known protective devices and systems define a high voltage ride through (HVRT) capability.
Most known main power transformers and auxiliary power transformers tend to transmit the associated voltage transients from the grid to the equipment. For the auxiliary electrical system, HVRT/LVRT/ZVRT capabilities include tap changer systems on the main power transformer and/or the auxiliary power transformer to regulate the voltage of the electric power transmitted from the grid to the auxiliary equipment. However, such changer systems are electromechanical and may not operate quickly enough to maintain the voltage to the auxiliary equipment in the 90% to 110% tolerance band. Also, such tap changer systems regulate the voltage in discrete, incremental steps and may not provide the voltage within the tolerance band to facilitate extended and continuous operation of the auxiliary equipment.